In applications of electron optics, it is often desirable to create small, bright focused spots. There are many factors that can limit the ability to finely focus an electron beam. Some of the more common ones include spherical and chromatic aberrations, variations in the mass or charge of the beam particles, magnified source size, misalignments of key components, mutual coulomb repulsion of the charged particles in the beam, inadequate magnetic and electrostatic shielding, mechanical vibrations, and deflection aberrations. This invention is primarily directed to the correction of deflection aberrations although some of the other aberrations will come into consideration since they are often linked in practical designs.
Electron beam probes having a diameter of a few Angstroms are possible, but only within a very small scanned field of a few hundred Angstroms. Most applications of electron beams, however, require moving the beam around appreciably more. When a beam is deflected, aberrations of deflection are induced. These deflection aberrations are usually significant and often much larger than the undeflected focused spot size. As those skilled in the art will appreciate, the above holds equally true for electron beams as well as for other charged particle streams.
There are two types of image defects that result from deflection. The first is a field defect that distorts rectangles into pincushion or barrel shapes. The second type of defect causes the focused spot to increase in size. The aberrations that cause the focused beam size to increase are of more concern. Depending on the application, an electron beam designer will usually want perimeter or corner resolution to be the same or at least not significantly worse than resolution at the center of focus. Because deflection aberrations are approximately proportional to the beam diameter and the square of the deflection angle, the designer will often compromise center brightness and resolution in order to make corner resolution and brightness acceptable. This is typically achieved by reducing the diameter of the beam in the deflection region. As a demonstration of the interdependence among the aberrations, this tends to increase space charge repulsion.
There are two ways to deflect an electron beam--transverse magnetic fields and transverse electric fields. One aberration of concern results from different parts of the beam experiencing different deflections due to inherent non-uniformities of magnetic or electric fields in a vacuum. For the same amount of deflection, the coefficient of deflection aberration is larger for electric field deflection as compared to magnetic field deflection. Electrostatic deflection can be modulated at a very rapid rate and requires low power, but electrostatic deflection aberrations can be several times worse than magnetic deflection aberrations. Researchers have tried to reduce electrostatic deflection aberrations since cathode ray tubes first became useful devices early in this century. (Electric field deflection is often called electrostatic deflection whether the activity is static or dynamic.) While some improvements have been demonstrated over the years, a major solution has yet to be satisfactorily identified.
For some applications, charged particle beams are scanned or dynamically deflected. For other applications the beams are deflected statically. Yet other applications include both static deflections and dynamic deflections. The above concerns apply equally to stable deflection, dynamic deflection and combinations of both.
The standard approach to studying deflection aberrations is to approximate solutions to complex integral equations using polynomial expansions of displacement or angle to third and higher order terms. As those skilled in the art will appreciate, these higher order calculations are enormously complicated and typically require equations that fill entire pages. The results of these calculations, however, show that deflection aberrations partially represent quadrupoles, and thus the net effect of deflection aberrations can be reduced by suitable introduction of another quadrupole of opposite polarity. Correcting quadrupoles are well known and are often called stigmators.
A quadrupole produces astigmatism. Astigmatism, unlike most other electron optical aberrations can be either negative or positive. Pure astigmatism can be exactly canceled by another quadrupole of opposite sign disposed elsewhere in the optical stream. The correcting quadrupole could be either magnetic or electrostatic, but should be adjustable in magnitude and orientation. Suitable quadrupole devices are generally known in the art. Quadrupoles can therefore reduce deflection aberrations for both magnetic deflection as well as electrostatic deflection.
In the case of electrostatic deflection, nonuniformities in the deflecting field appear most pronounced near the plate surfaces, and deflection aberrations are considered to be smallest in the exact center between two oppositely charged deflection plates. Because there is no preferred direction to scan the beam, the scan is typically performed equally toward either plate. Thus, in the art of electrostatic deflection, the scan is usually symmetrical and beams are centered between the plates. The distance from the beam to the end of the plates in the direction transverse to deflection is sufficient to prevent edge effect fields from perturbing the deflection. Typically, 0.25 to 0.5 inches is used in the art where beams are centered between the plates and scanned symmetrically using electrostatic deflection. In general, prior known asymmetrical scanning systems have not addressed the correction of deflection aberrations. One solution is disclosed in co-pending application Ser. No. 08/623,918, the contents of which are hereby expressly incorporated herein by reference.
Assuming that electrostatic deflection aberrations could be totally eliminated, as already indicated, that may not necessarily mean that deflected beams could be focused to an infinitesimally small spot. There are other contributing factors. Of special interest, among these other terms are chromatic aberrations and variations if any in the ratio of charge to mass of the particles. Electrostatic components are particularly known to have relatively high chromatic aberrations. Chromatic aberrations can limit performance in some important applications of electron optical systems and is often a consideration in practical product designs. These aberrations stem from thermal energy differences that exist among the electrons in a beam. These energy differences are on the order of kT, which is Boltzman's constant times the absolute temperature of the material from which the electrons are emitted. For room temperature of .apprxeq.300 degrees Kelvin, kT is approximately 0.02 electron volts. For indirectly heated oxide coated cathodes (.apprxeq.1100 degrees Kelvin), which are normally used in television tubes, the energy spread has a mean value of 0.1 electron volt. For directly heated tungsten filaments (.apprxeq.2500 degrees Kelvin), the energy spread mean is 0.2 volts. This energy spread can result in an appreciable chromatic aberration that limits performance for many applications.
It is not difficult to estimate the importance of these thermal energies. The thermal energy is randomly oriented and necessarily has transverse and longitudinal components. The transverse component is most troublesome. For example, a 20 kv beam is deflected 10 inches over a throw of 12 inches. Ignoring small relativistic effects at this voltage, the velocity of an electron is proportional to the square root of the energy of the electron. The angular spread of the 20,000 volt beam due to a transverse energy of 0.1 electron volt is therefore (0.1/20,000+L ) or 0.0022 radians. This angular spread increases to 0.027 inches after 12 inches of throw. Even if all other aberrations are completely eliminated, the thermal spread would cause spot size blur on the order of 0.027 inches.
In the above example, 0.1 volts added to 20,000 volts along the direction of the gun axis (longitudinal component of thermal energy) adds an insignificant 0.00002 inches to the spot size. Despite the fact that hotter cathodes are more copious emitters of electrons, this calculation shows why in critical applications--even those involving very small deflections--lower temperature electron sources are often used. This also shows that correcting chromatic aberrations would be advantageous once deflection aberrations are reduced in a high resolution electron optics system. Equally useful would be a way to filter out the low and high energy electrons from the beam. That would tend to make the electron optical system independent of chromatic aberrations.
CRT displays are important useful products that extensively make use of electron optical technology. Upon comparing the CRT to flat panel display alternatives, the major disadvantage of the CRT is the large horizontal surface area or footprint that the CRT occupies. It is therefore advantageous to reduce this surface area or CRT footprint wherever possible.
While the deflection of electron beams are discussed above, the above concerns apply to positive or negative ion beams as well. Charged ion beams are not used in display devices because of their relatively large inertia, but there are other uses of interest. The applications of ion beams where deflection aberrations are of concern are in lithography, mass separation and mass spectroscopy. In these applications, charged particle beams of finite diameter are deflected and it is desirable that their landing points are independent of where the particular particle is within the beam envelope.
In mass spectroscopy, there are two basic ways to separate ions of different mass. The first method is to determine the time of flight in a line of sight to a target that is oppositely charged compared to the particle. The acceleration and therefore the time of flight is dependent on the charge to mass ratio. This first method does not depend on deflection aberrations because there are no deflections.
The second method is to deflect the charged particle beam with a magnetic or electric field, or both magnetic and electric fields. Ignoring relativistic effects, the forces are proportional to the charge divided by the mass for either electric or magnetic deflection. Particles of different mass will land in different locations in proportion to mass/charge which may be measured and used to identify the ionic mass. The resolving power of a mass spectrometer is the minimum mass difference of the ions that can be determined. This can be limited by deflection aberrations. That is, if identically massive ions, which originate from different locations in the beam envelope, are deflected into different landings it can compromise the resolution of the mass spectrometer. Therefore, in mass spectrometry applications involving deflection of ion beams with magnetic or electric fields, deflection aberrations can be a concern.
It is also sometimes desired to separate a beam of ions of mixed masses. One example where this would be useful is to enrich the concentration of the fissionable uranium isotope U-235 relative to the more common isotope U-238, which is non-fissionable. Electrostatic deflection can be used to accomplish this separation because the ratio of charge to mass is approximately 1% different for these two isotopes. This process, however, would be limited by electrostatic deflection aberrations for the same reasons as discussed for the mass spectrometer.
The prior art lacks the advantage of electrostatic deflection of a charged particle stream without associated deflection aberrations. Overcoming the deflection aberrations inherent in electrostatic deflection systems would allow for greater resolution and control of the particle stream. Such control provides improved display quality for cathode ray tube displays such as televisions and computer monitors, as well as other uses of charged particle streams including a CRT with reduced footprint, an energy filter, a mass spectrometer, and a mass separator.